The present invention relates to a hydrogen storage alloy electrode capable of electrochemically absorbing and desorbing hydrogen, a method for producing the electrode, and an alkaline storage battery comprising the electrode.
Conventionally, various types of metals and alloys, such as Cd used for the negative electrodes of nickel-cadmium storage batteries, Fe and Zn, have been used for the negative electrodes of alkaline storage batteries. Among them, hydrogen storage alloys have been practically used for the negative electrodes of nickel-hydrogen storage batteries.
For the current nickel-cadmium storage batteries, rare earth-nickel based alloys have been mainly used. Regarding alloys other than rare earth-nickel based alloys, research of alloys of a group called Laves phase alloys has been made so as to achieve negative electrodes with higher capacity. Moreover, research has been made for alloys such as Vxe2x80x94Tixe2x80x94Ni based alloys that are produced by using V and T, which show excellent hydrogenation properties even when used alone, as bases and adding Ni to impart electrochemical activity for use in batteries.
However, these alloys have not yet been put into practical application in view of total performance of batteries including life characteristics and storage characteristics.
On the other hand, an AB5 type alloy is given as one of main practically used alloys. This AB5 type alloy is an alloy among rare-earth metal alloys, composed of an element having strong hydrogen affinity (A element), such as a rare-earth element, and an element (B element) having week hydrogen affinity, such as transition element, in a stoicheiometric ratio (A:B) of 1:5, and has a CaCu5 type crystal structure of hexagonal system.
Conventionally, research and development have being made for this AB5 type alloy among the rare-earth based alloys, and a variety of alloys having a wider range of compositions compared to compositions depending on the stoicheiometric ratio have been proposed (for example, Japanese Laid-open Patent Publication No. Sho61-91863, and the like). Among alloys having such a non-stoicheiometric composition ratio, an alloy which is rich in A is proposed as an alloy having a particularly high capacity (for example, Japanese Laid-open Patent Publication No. Hei2-27737, the description of Japanese Patent No. 2680623, and the like).
Furthermore, the description of Japanese Patent No. 2680628 discloses a hydrogen storage alloy electrode composed of a mixture of a hydride reactive with oxygen preferentially to the above alloy and not concerned with charge/discharge reaction of an electrode and an alloy having a CaCu5 type crystal structure. According to this hydrogen storage alloy electrode, it is possible to limit oxidation and deterioration of the hydrogen storage alloy contributing to a charge/discharge reaction of the electrode and preliminarily charge a negative electrode. It is therefore possible to improve the cycle characteristics.
Besides, as rare-earth based alloys other than the AB5 type alloy, an alloy (A2B7 type alloy) with the A:B ratio of 2:7 has also being a subject of research and development (for example, Japanese Laid-open Patent Publications Nos. Hei9-194971, Hei10-60565, and the like). It has been known that this A2B7 type alloy is transformed to amorphous when it is hydrated, and a battery disclosed in Japanese Laid-open Patent Publication No. Hei10-60565 has reached a level for practical application although it is relatively difficult to control the discharge characteristics and ensure the life characteristics.
In addition, the specification of U.S. Pat. No. 5,840,166 discloses hydrogen storage alloy electrodes whose performance was improved by mainly using light rare-earth (La, Ce, Pr, Nd) as the A element in a wide range of alloys with the A:B ratio of 1:3 to 1:4 and adding heavy rare-earth (such as Gd) or Y, Sc, Mg, Ca so as to contain anti-phase boundaries extending perpendicular to the C-axis of a crystal grain particularly.
Since the Co contained in the above-described rare earth-nickel based alloys is a rare resource and expensive, rare earth-nickel based alloys having compositions with a reduced Co content have been desired for a reduction in the costs of alkaline storage batteries.
However, when a hydrogen storage alloy mainly composed of a rare-earth based alloy is used for an alkaline storage battery, whether Co is substantially contained in the alloy is an important factor in improving corrosion resistance to an electrolyte and limiting pulverization of the alloy caused by cycle deterioration.
Hence, it is the first object of the present invention to solve the above-described problems and reduce the costs by containing a little or no Co in the hydrogen storage alloy electrode of an alkaline storage battery. The second object is to improve the corrosion resistance to an electrolyte and the life characteristics of the battery by limiting pulverization of the alloy caused by deterioration of cycles.
The present invention relates to a hydrogen storage alloy electrode composed of a hydrogen storage alloy which has a CaCu5 type region and a Ce2Ni7 type region in the crystal structure thereof and satisfies the relational formula: p:q=1:(4+a), where p is the sum of the mole fraction of an element occupying the Ca site of the CaCu5 type region and the mole fraction of an element occupying the Ce site of the Ce2Ni7 type region, q is the sum of the mole fraction of an element occupying the Cu site of the CaCu5type region and the mole fraction of an element occupying the Ni site of the Ce2Ni7 type region, and xe2x88x920.2xe2x89xa6axe2x89xa60.4.
In this case, it is preferred that the hydrogen storage alloy is represented by the compositional formula: R(Ni1xe2x88x92yMny)4+aMb, where R is at least one kind of La, Ce, Sm, Nd, Pr and misch metal, M is at least one kind of Fe, Cr, Cu and Al, 0.05xe2x89xa6yxe2x89xa60.4, xe2x88x920.2xe2x89xa6axe2x89xa60.4, 0xe2x89xa6bxe2x89xa60.4, and a+bxe2x89xa60.4.
Moreover, it is preferred that the hydrogen storage alloy contains 1 to 10% by weight of L, where L is at least one kind of Ti, V, Zr, Nb, Mo, Si and Ge.
Furthermore, it is preferred that particles of Co are present on a surface of the hydrogen storage alloy.
Besides, the present invention also relates to a method for producing a hydrogen storage alloy electrode, comprising: (a) preparing components satisfying the compositional formula: R(Ni1xe2x88x92yMny)4+aMb, where R is at least one kind of La, Ce, Sm, Nd, Pr and misch metal, M is at least one kind of Fe, Cr, Cu and Al, 0.05xe2x89xa6yxe2x89xa60.4, xe2x88x920.2xe2x89xa6axe2x89xa60.4, 0xe2x89xa6bxe2x89xa60.4, and a+bxe2x89xa60.4; (b) melting the components and synthesizing and obtain a hydrogen storage alloy by an atomization method, a roll rapid quenching method or a centrifugal spraying method; (c) heat-treating the hydrogen storage alloy in a vacuum or in an inert gas at a temperature between 500xc2x0 C. and 1065xc2x0 C.; and (d) joining the hydrogen storage alloy and a support integrally to form an electrode.
In this case, after step (c) but before step (d), it is preferred to immerse-treat the hydrogen storage alloy in an alkali solution, which has a temperature between 80xc2x0 C. and a boiling point and contains a cobalt oxide, cobalt hydroxide or cobalt salt added thereto.
Further, it is preferred to comprise the step of adding 1 to 10% by weight of L, where L is at least one kind of Ti, V, Zr, Nb, Mo, Si and Ge, to the hydrogen storage alloy.
In addition, the present invention relates to an alkaline storage battery comprising: a negative electrode mainly composed of the above hydrogen storage alloy; a positive electrode mainly composed of nickel hydroxide; a separator formed of non-woven fabric, for example, which electrically separates the negative electrode and the positive electrode; an alkaline electrolyte; and a sealed container with a safety valve.
In this case, it is preferred that particles of Co are present on a surface of the hydrogen storage alloy.
Further, it is preferred that the hydrogen storage alloy contains 1 to 10% by weight of L, where L is at least one kind of Ti, V, Zr, Nb, Mo, Si and Ge.